Anxiety and school fears are relatively frequent in childhood. Psychology and education professionals need to have assessment instruments for screening for school anxiety in schools. This study aimed to develop, adapt, and examine the reliability and validity evidence of the School Anxiety Inventory for Primary Education (SAI-PE) scores. Using random cluster sampling, a sample of 843 Spanish students (51.7% girls), ages ranging from 8 to 12 years (M = 10.03, SD = 1.25), was selected from public and concerted schools. The content validity of the SAI-PE scores was analyzed through experts' judgment and students' assessment of their comprehension of the instrument's items. Exploratory and confirmatory factorial analyses support the validity of the SAI-PE scores. The results revealed a multifactorial structure for the scores of anxiety reactions (cognitive, psychophysiological, and behavioral) and school situations scores (school punishment, victimization, social, and academic evaluation). The study conclusions reinforce the importance of having instruments that assess the complexity of emotional responses such as school anxiety, favoring its early detection and evaluating the effectiveness of preventive and/or therapeutic programs in schools.